1. Field of the Invention
This invention relates to a sheet cutter for cutting a sheet, particularly to one having a motor driven push cutter for cutting a portion of a continuous sheet discharged from a printer such as for hand held electric appliances.
2. Description of the Related Art
A role of paper used in a printer of some hand held electric appliances, such as a terminal register used in restaurants, is required to be cut bilaterally, namely, sometimes a printed part of the paper is cut off completely as a receipt for a customer and at other times a plurality of printed parts are cut incompletely between the successive printed parts as a series of receipts for a group of respective customers, the plurality of printed parts remaining connected by a central uncut part of narrow width. Each of the printed parts can be easily separated from each other by hand later on if every customer wants to have his. The former is often call full cut and the latter, half cut.
FIG. 1 is an exploded view of a main part of a prior art sheet cutter, and FIG. 2 is a front view of the main part of the prior art sheet cutter, which is looked at along an arrow indicated by a letter A. The sheet cutter 1 comprises a driving unit, or member, 11 mounted on a side wall of a chassis 1a, a rotating disc 12 assembled in the driving unit 11, a moving knife unit 13 making a moving knife 131 moved upwardly and downwardly by mounting both on the side wall of the chassis 1a and the rotating disc 12, and a stationary cutter unit 14 having a stationary knife 141 on it opposed to the moving knife 131. The driving unit 11 further includes a gear wheel 111 freely rotatable around a pivot P (i.e., an axle, or a shaft) mounted on a side wall of the chassis 1a, a worm gear 115 engaging with the gear wheel 111, and a driving motor 114 mounted on the side wall of the chassis 1a, a drive shaft of which is directly connected with an axle (i.e., a shaft) of the worm gear 115. Both a rotation pivot (i.e., shaft) 112, for the rotating disc 12, and a first sliding pin 113, restricting an angle of rotation of the rotating disc 12, are formed on respective peripheral positions of the gear wheel 111, projecting from an exposed side of the gear wheel 111. The rotating disc 12, having nearly the same diameter as that of the gear wheel 111, has a pivot bore 12a penetrated by the pivot 112 and an arcuate slit 12b for receiving therein and allowing the first sliding pin 113 to move therein, and which limits the angle of rotation of the rotating disc 12, and a second sliding pin 12c projecting from the exposed side of the gear wheel 111 into an elongated slot, or base, 123b of link arm 132 and which propagates driving forces to the moving knife unit 13. Thus, the rotating disc 12 is rotatable within the limited angle defined by the arcuate slit 12b, relatively to the gear wheel 111, when the rotating disc 12 is mounted on the gear wheel 111 by engaging the pivot 112 with the pivot bore 12a and the first sliding pin 113 with the slit 12b. The moving knife unit 13 comprises a moving knife 131 and the above-noted link arm 132, jointed with each other. The moving knife 131 has a vertical arm 131d on the upper side, an angle knife 131b with a setback area of a square notch 131a at the central region S on the lower side, and a pair of legs 131c. The link arm 132 has a pivot bore 132a on a first end engaged with a pivot 1a' protruding from the side wall of the chassis 1a and a long bore 132b engaged with the second sliding pin 12c of the rotating disc 12 on a second end. The link arm 132 is jointed with the moving knife 131 by a pin 132c (FIG. 2) at the upper end of the vertical arm 131d such that the link arm 132 is rotated around the pin 132c in a plane.
The stationary knife unit 14 comprises a knife stage 142 and a stationary knife 141 mounted on the knife stage 142, in which the stationary knife 141 is arranged in the foreground relative to the moving knife 131 in FIG. 1. The knife stage 142 has a narrow groove 142a into which the moving knife 131 is inserted. The narrow groove 142a has a pair of guiding portions 142b on both ends, which guide the respective legs 131c. A printed portion of continuous paper 15 is discharged through a gap between knife edges of the moving and stationary knives in the foreground direction indicated by a letter B.
FIGS. 3 and 4 are front views of the main part of a sheet cutter of a first prior art device, to explain full-cut and half-cut operations, respectively. In FIG. 3, the gear wheel 111 rotates clockwise about its axis, or pivot, P as indicated by a letter B.sub.1 by the normal rotation of the motor 114, which causes a pivotal motion of the rotating disc 12 in the direction of an arrow as indicated by a letter C.sub.1 by reactions from the first sliding pin 113 to the slit 12b and from the long bore 132b to the second sliding pin 12c. The reverse rotation of the motor 114 causes a counter clockwise rotation of the gear wheel 111 as indicated by a letter B.sub.2 and then a pivotal motion of the rotating disc 12 in the direction of an arrow as indicated by a letter C.sub.2 as shown in FIG. 4. Since a clockwise rotating radius R.sub.1 of the rotating disc 12 is larger than a counter clockwise rotating radius R.sub.2, a down stroke of the moving knife 131 for clockwise rotation of the rotating disc 12 is longer than that for counter clockwise rotation of the rotating disc 12. The longer stroke of the moving knife 131 cuts off the discharged portion of continuous paper completely, while the shorter stroke cuts it incompletely by stopping the moving knife 131 such that the setback area of a square notch 131a at the central region on the lower side of the moving knife 131 shown in FIG. 1 is maintained above the upper surface of the paper. Therefore, the full-cut and half-cut of the paper can be carried out by selecting the normal and reverse rotations of the motor 114, respectively.
FIG. 5 is a front view of the main part of a prior art sheet cutter to explain a first drawback. The first drawback of the prior art is the fact that since the link arm 132 jointed with the moving knife 131 by the pin 132c is pivoted at 132a, a total force applied to the pin 132c denoted by a letter F.sub.1 has a horizontal component of force when the moving knife 131 is pushed down. Therefore, the horizontal component of force causes an angular momentum in a plane including the moving knife which eventually often hinders the moving knife from moving down smoothly.
FIG. 6 is a front view of the main part of a prior art sheet cutter to explain a second drawback. The second drawback of the prior art is the fact that since the square notch 131a having a width denoted by a letter b at the central region on the lower side of the moving knife 131 has no sharp knife edge, a cross-section of the paper is not as sharply-cut in the fall-cut operation, wherein the cutting portion at the central region is protruded by a length corresponding to thickness of the knife edge denoted by a letter t from the other cutting portions in both sides 15a of the paper 15, and wherein, in the half-cut operation, the uncut portion having width 15b causes an irregular cutting shape when it is torn off by hand, which is not favorable in appearance.